In recent years, reduction of carbon dioxide has been strongly demanded in order to deal with air pollution and global warming. In the automobile industry, the reduction of carbon dioxide emission due to the introduction of electric vehicles (EVs) or hybrid vehicles (HEVs) has been expected, and electrical devices such as a secondary battery for motor driving, which is a key to achieving the practical application of these vehicles, have been developed extensively.
The secondary battery for the motor driving has been required to have an extremely high output characteristic and high energy as compared to a lithium ion secondary battery for consumer use that is used for a mobile phone, a laptop computer, or the like. Therefore, the lithium ion secondary battery with the highest theoretical energy among all the batteries has attracted attention, and the development thereof has been advanced rapidly.
The lithium ion secondary battery generally has a structure in which a positive electrode formed by applying a positive electrode active material or the like on both surfaces of a positive electrode current collector using a binder and a negative electrode formed by applying a negative electrode active material or the like on both surfaces of a negative electrode current collector using a binder are connected to each other through an electrolyte layer, and housed in a battery case.
Conventionally, a carbon/graphite-based material has been used for the negative electrode of the lithium ion secondary battery because such a material is advantageous in cost and lifetime of the charging-discharging cycle. However, in the case of using the carbon/graphite-based negative electrode material, charging and discharging are performed by intercalating and deintercalating lithium ions in and out of graphite crystals; thus, a charging-discharging capacity of more than or equal to the theoretical capacity, 372 mAh/g, that is obtained from LiC6 corresponding to the maximum lithium introduction compound cannot be achieved. This is disadvantageous. In view of this, it has been difficult to achieve the capacity and the energy density that satisfy the practical application level of the carbon/graphite-based negative electrode material for the use in the vehicle.
On the other hand, the battery including a material that is alloyed with Li in the negative electrode, which has higher energy density than that of the conventional carbon/graphite-based negative electrode material, has been anticipated as the negative electrode material for the use in the vehicle. For example, a Si material intercalates and deintercalates 3.75 mol of lithium ions per mole of the Si material in charging and discharging as expressed in the following Reaction Formula (A), and the theoretical capacity is 3600 mAh/g in Li15Si4 (=Li3.75Si).Si+3.75Li++e−⇄Li3.75Si  (A)
However, in the lithium ion secondary battery including the material that is alloyed with Li in the negative electrode, the negative electrode expands and contracts largely in charging and discharging. For example, when Li ions are intercalated, the negative electrode expands in volume by about 1.2 times in the case of the graphite material; on the other hand, in the Si material, when Si and Li are alloyed, the amorphous state transits to the crystal state and the volume changes largely (about four times), and therefore, the cycle lifetime of the electrode deteriorates, which is a problem. In addition, in the case of the Si negative electrode active material, the capacity and the cycle durability are in the trade-off relation, and it has been difficult to improve the cycle durability while the high capacity is exhibited, which is a problem.
Here, International Publication No. WO2006/129415 discloses the invention whose object is to provide a nonaqueous electrolyte secondary battery including a negative electrode pellet with high capacity and excellent cycle lifetime. Specifically, silicon powder and titanium powder are mixed by a mechanical ironing method and crushed in a wet process so that a silicon-containing alloy is obtained. The silicon-containing alloy including a first phase mainly containing silicon and a second phase containing titanium silicide (such as TiSi2) is used as the negative electrode active material. This literature also discloses that, in this case, at least one of these two phases is amorphous or low-crystalline.